fix(upload): reject exact duplicates at upload time and prevent duplicate mobile uploads

- Move duplicate check before task enqueue in ui_upload endpoint
- Clean up temp file and return status "duplicate" for exact duplicates
- Add URI-level dedup guard in mobile UploadScreen to prevent repeated uploads
- Improve ShareContext URI normalization (collapse slashes, decode percent-encoding)
- Guard +not-found.tsx effect against re-firing for the same pathname
- Update mobile UploadResponse type and handlers for duplicate status
- Update web frontend upload.js to show duplicate status
- Update API and Configuration docs

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/src/services/api.ts

@@ -66,10 +66,16 @@ export interface FileRecord {
 }
 
 export interface UploadResponse {
-  task_id: string;
+  task_id?: string;
   status: string;
   original_filename: string;
   stored_filename: string;
+  duplicate_of?: {
+    duplicate_type: string;
+    original_file_id: number;
+    original_filename: string;
+    message: string;
+  };
 }
 
 // ---------------------------------------------------------------------------